Method of preparing siloxane polymers utilizing silazanes



Patented Apr. 20, 1954 METHOD OF PREPARING SILOXANE POLY- MERS UTILIZING SILAZANES John L. Speier, Pittsburgh, Pa., assignor to Dow Corning Corporation, Midland, Mich., a corporation of Michigan No Drawing. Application September 3, 1952,

Serial No. 307,704

2 Claims. (01. 260-465) 1 This invention relates to a method of prepar ing polymers by reacting a silazane with a hydroxylated organosilicon compound and to the copolymers so prepared.

Previously siloxanes have been polymerized by hydroxyl condensation and by siloxane bond rearrangement. In both cases elevated temperatures and catalysts are required if the process is to be carried out at a feasible rate. In such cases the catalyst remain in the finished polymer. This may cause undesirable reactions later due to degradation of the polymer at elevated temperatures. It would be highly desirable there fore to polymerize siloxanes by a method which leaves no residual catalyst. Attempts to do this by the condensation of silicon bonded hydroxyls by heat alone does not produce satisfactory products.

The applicant has solved this diiiiculty by a novel method of reacting hydroxylated organesilicon compounds with organosilazanes. The reaction proceeds smoothly even at room temperature with the elimination of ammonia or methyl amine. The resulting products are free of undesirable catalysts and have unique properties. The resinous polymers prepared by this method are particularly characterized by a combination of hardness and toughness which is not exhibited by organosiloxanes polymerized by conventional methods. This invention is particularly adapted for use in the preparation of potting compounds for electrical apparatus. I

In accordance with this invention, an organosilicon compound containing at least .6 per cent by weight silicon bonded hydroxyl groups and from .9 to 3 hydrocarbon radicals per silicon atom, the remaining valences of the silicon, if any, being satisfied by oxygen atoms, is reacted with an organosilazane of the formula The silazane is employed in amount of at least .1 per cent by Weight of the hydroxylated organosilicon compound.

The hydroxylated organosilicon compounds which are employed in this invention include both monomeric materials and polymeric ma terials. Any organosilicon compound containing the required amount of hydroxyl groups in which the organic radical attached to the silicon are monovalent hydrocarbon radicals free of (EEG linkages is operative in this invention. Thus the hydroxylated organosilicon compoundsziuclude monomeric silanols such as RsSiOH and R2Si(OH2, polymers of the formula HO (RzSiO) xH tion of these radicals may be present in the siloxane. 1

The silazanes which are operative in this'invention are of the formula (R,,SiNR a:

where R is a monovalent hydrocarbon radical free of C50 linkages, n has a value from 1.99 to 2, R. is hydrogen or methyl and a: is an integer of at least 3. These silazanes are polymeric materials having at least 3 polymer units per molecule. These materials may be best prepared by reacting the corresponding chlorosilane with liquid ammonia or With liquid 'methyl amine in the manner described in the applicants copending application filed concurrently herewith. For the purpose of this invention; the R groups on the silazane may be, for example, methyl, ethyl, octadecyl, phenyl, tolyl, xenyl, benzyl, allyl, vinyl, hexadienyl, cyclohexenyl and cyclohexyl or any combination thereof. The silazanes are com posed essentially of units of the formula RzSiNR but may also contain small amounts of unit 'oi the structure v N (RSiN) N The process of this invention proceeds spontaneously upon mixing the above hydroxylated compounds with the silazanes. The schematic equation. is:

a v Qe'eSiOH SiNSi ZESiOSiE R'NH; Reaction proceeds even below room temperature and may be carried out at temperatures ranging up' to 400 C. However, in view of the rapidity of'thereaction and the superior products obtained' there is little advantage in operating at temperatures'above C. In order to realize the benefits of this invention, the silazane should be employed in amount of. at least .1 per cent by p weight based on the hydroxyla-ted organosilicon compound. Any amount of silazane above the minimum maybe employed if desired. In those cases in which the amount of silazane is such that there is more than one SiNSi group per two OH groups, some of. the silicon" atoms in the resulting polymer will be connected by SiOSi linkages and the remaining silicon atoms will be connected through SiNSi linkages. These co= polymers of siloxanes and silazanes exhibit some unique properties. They are rubbery in character resembling the gels obtained by cross-linking siloxanes and yet the silazane copolymers are soluble. These copolymers-exhibit a rust inhibiting effect when applied to steel. When the SiNSi linkages are exposed tomoisture they hydrolyze with the evolution of ammonia. The resulting product is a siloxane.

The siloxane-silazane copolymers Within the scope of this invention are composed of from 26- to 99.99 mol per cent si'loxa-ne units of the formula and from .01 to '75 mol per cent silazane" units monoorganoand trioi'ganosiloXane units may have structures as indicated below.

The method of this invention isparticularly adaptable to the preparation of superior" coating compositions. This is illustrated by the fact that a direct comparison between a siloxane resin which is set by conventional zinc naphthanate dryers requires higher temperatures than the same composition which is polymerized by reaction with a silazane. Furthermore, the finished coating prepared by the present novel method exhibits more toughness and hardness than the coating prepared by conventional methods. In addition the process of this invention readily adapts itself for the potting of electrical equipment. When suitable siloxane compositions are mixed with the silazanes and poured into a space around electrical equipment, the siloxane' will then polymerize to a gel in a short length of time at low temperatures and'the 'by-produced arnmoni'aescapes without bubble formation. Thus there is no residue catalyst to further alter the properties of the product. Such potting compositions retain their resilience and dielectric strength from very low to very high temperatures.

The following examples are illustrative of this invention. butare not to be construed as limiting the scope thereof. The latter is properly delineated in the appended claims.

Example 1 6-0 g. of a siloxane resin having the composition 33 mol per cent dimethylsiloxane, 33 mol per cent 'monomethylsiloxane, and 33 mol per cent monophenylsiloxane' and containing about 3.5 per cent'by weight silicon bonded 0H groups, was dissolved in toluene to give a 60 per cent solution. 10' g. of hexamethylcyclotrisilazane was added to the solution and films thereof were cast on a glass plate. The composition gelled in 2 hours at room temperature and was then heated 2 hours at Cf.- The resulting films were stripped from the glass and were found to have superior physical strength to the other silicone films which had necessarily been prepared at much higher temperatures.

Example 2 A mixture of 2.5 g. of a hydroxylated copolymer composed of 70 mol percent dimethylsiloxane and 30 mol per cent monophenylsiloxane was mixed with 2.5 g. of a hydroxyl end-blocked phenylmethylsiloxane fluid. The resulting mixture' contained 5 per cent silicon bonded hydroxyl groups. .6 g. of hexamethylcyclotrisilazane was added to the mixture and the resulting material was poured around an electrical resistor in a small beaker. The mixture was heated overnight at 70 C. whereupon it had set to a resilient material. The system was alternately heated to C. and cooled to 50 C. without any deleterious effects on the dielectric properties of the siloxane.

Example 3 10 g. of tetramethyldisiloxanediol' was mixed with 10 g. of hexamethylcyclotrisilazane and heated to 67-70 C. for 72 hours. The resulting product was a copolymer containing both SiDSi and SiNSi linkages. This material was a rubbery polymer.

Example 4 g. of a hydroxylated dimethylsiloxane fluid containing 3.22 per cent silicon bonded hydroxyl groups was mixed with 13.5 g. of hexamethylcyclotrisilazane and heated 6 hours at 158-71 5? 6. Arubbery polymer was obtained which was found by analysis to contain .44 per cent nitrogen in the form of v r 1 sag-silinkag'es.

v 7 E.'!:am1;ple o 7 291 g. of a dimethylsiloxane ,fiuid containing 6' 3.2 per cent silicon bonded OH groups was mixed with 51 g. of-a mixture othexamethylcyclotrisilazane and a trace oi a monomethylsllazane oi the unit structure N C HAP-N) N and heated at 115 C. for 13 hours. A clear rubbery material was obtained which contained both siloxane and silazane linkages.

Example 7 50 g. of a siloxane resin containing 28 mol per cent dimethylsiloxane, 34 mol per cent monomethylsiloxane and 38 mol per cent monophenylsiloxane and containing 3.5 per cent by weight silicon bonded hydroxyl groups, was mixed with 9.46 g. of hexamethylcyclotrisilazane and coated on a glass plate. After 15 minutes at room temperature, the resin had set. After heating 1 hour at 60 C., the film had a pencil hardness of between B and 33. Another sample was dipped and heated 1 hour at 140 C., at the end of that time the pencil hardness was 2H. By contrast, a sample of the siloxane resin having the composition shown above but containing zinc naphthanate as a catalyst, gave a pencil hardness of only 53 when applied to glass and heated 1 hour at 60 C. The same resin had a hardness of only 33 when heated 1 hour at 140 C. on glass. These tests show the greater rapidity of hardening obtained by the method of this invention. Furthermore, the films obtained by the silazane polymerization were not only harder but tougher and more flexible than those obtained by the conventional method.

Example 8 When 2 g. of [(CH3)2SiNCH3]3 is reacted with g. of the hydroxylated fluid of Example 5 and the mixture is heated at 60 C. for 2 hours, a viscous polymeric material is obtained.

Example 9 When 100 g. of a hydroxylated siloxane copolymer containing 2 per cent silicon bonded OH groups and having the composition mol per cent vinylsiloxane, 25 mol per cent dibutylsiloxane, 15 mol per cent cyclohexylsiloxane, 15 mol per cent xenylsiloxane and 30 mol per cent tolylmethylsiloxane is mixed with 25 g. of mixed silazanes of the formula H uHuX Im L where x is above 3, a resinous copolymer containing both siloxane and silazane linkages is obtained.

That which is claimed is:

1. A method of polymerizing a hydroxylated organosilicon compound containing at least .6 per cent by weight silicon bonded hydroxyl groups and from .9 to 3 monovalent hydrocarbon radicals free of C50 linkages per silicon atom, in said organosilicon compound any remaining valences of the silicon being satisfied by oxygen atoms, which method comprises jreacting said organosilicon compound with an organosilazane of the formula where R is a monovalent hydrocarbon free of CEO linkages, n has a value of from 1.99 to 2, R is selected from the group consisting of hydrogen atoms and methyl radicals and a: is an integer of at least 3, said silazane being present in amount of at least .1 per cent by weight based on the weight of the hydroxylated organosilicon compound.

2. A method of polymerizing a hydroxylated organosilicon compound containing at least .6 per cent by weight silicon bonded hydroxyl groups and from .9 to 3 hydrocarbon radicals selected from the group consisting of phenyl and methyl radicals, per silicon atom, in said organosilicon compound any remaining valences of the silicon being satisfied by oxygen atoms, which method comprises reacting said organosilicon compound with an organosilazane of the formula References Citcd in the file of this patent UNITED STATES PATENTS Name Date Patnode Apr. 11, 1950 Number 

1. A METHOD OF POLYMERIZING A HYDROXYLATED ORGANOSILICON COMPOUND CONTAINING AT LEAST .6 PER CENT BY WEIGHT SILICON BONDED HYDROXYL GROUPS AND FRROM .9 TO 3 MONOVALENT HYDROCARBON RADICALS FREE OF C=C LINKAGES PER SILICON ATOM, IN SAID ORGANOSILICON COMPOUND ANY REMAINING VALENCES OF THE SILICON BEING SATISFIED BY OXYGEN ATOMS, WHICH METHOD COMPRISES REACTING SAID ORGANOSILICON COMPOUND WITH AN ORGANOSILIAZANE OF THE FORMULA 